Materials that perform electromagnetic interference (EMI) shielding functions are useful in a variety of electronic components, cables, assemblies, and other devices. These materials usually provide EMI shielding by reducing or eliminating the passage of electromagnetic radiation therethrough. In general, the EMI shielding effectiveness of a material increases with increasing electrical conductivity, especially at low frequency. In view thereof, highly conductive materials such as metallic sheets, meshes, foams, and other metallic materials have traditionally been used for EMI shielding. However, most metallic materials are heavy and/or difficult to process, which increases manufacturing cost and limits design flexibility.
One possible alternative to current EMI shielding materials includes a conductive polymeric composite. Certain conductive polymeric composites are electrically conductive and may be processed via traditional polymer processing techniques, such as extrusion and injection molding. However, there are challenges associated with using conductive composites for EMI shielding applications. For example, in most instances, increasing filler loading simultaneously increases conductivity and decreases mechanical properties. The decrease in mechanical properties associated with the increase in conductivity creates a tradeoff between processability/mechanical strength and EMI shielding effectiveness.
Additionally, polymeric composites typically exhibit a complex dependence on frequency. The insulating nature of the polymer component of the composites results in lower conductivity of the composites compared with the metals, and subsequently lower shielding effectiveness in the low frequency range. Micrometer-scale conductive particle fillers are embedded in the polymer leading to low-conductivity apertures on the micrometer length scale. The exact size, shape, and orientation of the particles influences the distribution of low-conductivity apertures, which in turn influences the shielding effectiveness in the high-frequency range. In some cases, the micrometer-scale particles provide the advantage of smaller apertures compared to those in conventional metal shields such as braids, leading to higher shielding effectiveness in the high frequency range. In other cases, the apertures lead to very poor shielding effectiveness in the high frequency range. The frequency dependence of shielding performance in polymeric composite materials may be exacerbated in thin geometries, which is particularly challenging as even the shielding effectiveness of homogeneous materials decreases monotonically with decreasing thickness.
A composite formulation and a composite article that show one or more improvements in comparison to the prior art would be desirable in the art.